Non-uniform wire-grid lens antenna



R. L. TANNER Feb. 8, 1966 NON-UNIFORM WIRE-GRID LENS ANTENNA 4 Sheets-Sheet 1 "Filed Feb. 23, 1962 INVENTOR. 2/ ROBERT L.TANNER ATTORNEY R. L. TANNER Feb. 8, 1966 NON-UNIFORM WIRE-GRID LENS ANTENNA 4 Sheets-Sheet 2 Filed Feb. 23, 1962 INVENTOR. ROBERT L.TANN ER R. L. TANNER Feb. 8, 1966 NON-UNIFORM WIRE-GRID LENS ANTENNA 4 Sheets-Sheet 5 Filed Feb. 23, 1962 w m tumuim INVENTOR. ROBERT L. TANN ER BY u g. 1mm- ATTORNEY Feb. 8, 1966 R. L. TANNER NON-UNIFORM WIRE-GRID LENS ANTENNA Filed Feb. 23, 1962 4 Sheets-Sheet 4 ROBERT L.TANN ER ATTORNEY United States Patent Gffice Patented Feb. 8, 1956 3,234,557 NDN-UNIFORM WIRE-GEE) LENS ANTENNA Robert L. Tanner, 4780 Alpine Road, Menlo Park, Calif.

Filed Feb. 23, 1962, Ser. No. 175,370

6 Claims. (Cl. 343753) This invention relates to lens type antennas, and more particularly to a broadband non-uniform wire-grid lens antenna for operating over bands lying within the frequency range extending from below 1 to above 1,000 megacycles per second (me).

In my cop-pending application, Serial No. 175,369, filed on February 23, 1962, for Uniform Wire-Grid Lens Antenna, there is described a lens antenna comprising a pair of spaced, overlying, conductive wire-grids, in which the individual meshes are substantially identical in shape and size and in which the size of the meshes is substantially smaller than the shortest operating wavelength. As fully explained in the above referred to co-pending application, as long as the mesh size is small compared with the shortest operating wavelength the wave propagation characteristic is substantially independent of the operating frequency and is substantially isotropic.

The wave propagation velocity through the grid-wire lens as described in the above referred to co-pending application, and assuming a mesh size which is small in relation to the shortest operating wavelength, is changed by changing the spacing between opposite wire-grids from a distance which is small compared with the mesh size to a distance which is large compared with the mesh size. By properly varying the point-to-point spacing between the wire-grids, and thereby the point-to-point Wave velocity of propagation, a large variety of lens having different characteristics may be constructed. A very important lens so constructed is one having the property of converting a point source of HF energy from a simple feed antenna into a cophasal wave front, the equivalent of the optical Luneburg lens.

Generally, the wire-grid lens antenna described in my above referred to co-pending application has certain very important properties which make it ideally suitable for broadband operation. As long as the shortest operating wavelength is large compared to the mesh size, say 6 times, so that the transmission characteristic of, the space between the wire-grids is substantially independent of the, operating frequency and depends solely on theirspacing the wave velocity of propagation is substantially equal to /2 of the free space wave velocity for a wire-grid spacing of approximately one-tenth of the mesh size and equal to the free space wave velocity for a wire-grid spacing of approximately four times the mesh size.

Consequently, for a circularly symmetric antenna in which a variation of the effective permittivity from 2.0 at the center to 1.0 at the periphery is desired, the distance of separation (wire-grid spacing) must be varied from about one-tenth of the mesh size at the center to about four times the mesh size at the periphery. Accordingly, the distance of separation at periphery is about 40 times the distance of separation at the center. For example, the Luneburg-type wire-grid lens constructed in accordance with the invention disclosed in my co-pending application, operable over a frequency range from 4 to 30.mc. using a 5 ft. square-mesh wire-grid of No. 8 Wire, and having a diameter of 600 ft. has been found to require a distance of separation equal to 10.3' inches at the center of the lens and a distance of separation of33.0 feet at the periphery. If a biconical horn is now added to the 33 ft. peripheral opening it is easily seen that a very large horn structure is required and also that rather high support poles are necessary to support both the lens and the radiating horn.

In certain applications such a large variation of the distances of separation between opposite wire-grids to make use of the full range of wave velocities of propagation has been found undesirable. In certain other instances, the large physical separation required for obtaining free space wave propagation velocities has been found cumbersome and even though a decrease in mesh size could correct this condition, the added weight and cost for a smaller mesh sized wire-grid is often undesirable.

It is therefore a primary object of this invention to provide a broadband, lowside-lobe and low back-lobe lens antenna of the wire-grid type for an operating frequency range lying within the frequency range from below 1 mc. to substantially above 1,000 mc., in which the distance of separation of the spaced wire-grids for propagating a wave with a velocity equal to that of the free space wave velocity may be kept at value smaller than four times mesh size.

It is'a general object of this invention to provide an improved wire-grid lens antenna particularly suitable for use in the HF and VHF frequency ranges, which relative to its performance is inexpensive to construct and install, and'lighter in weight and smaller in physical size than wire-grid lens antenna known heretofore.

It is another object of this invention .to provide a broadband wire-grid lens antenna of thestationary type in which the propagation characteristic may be changed in a novel manner permitting the distance of separation between opposite wire-grids to be kept at a minimum.

It is a further object of this invention to provide a broadband wire-grid lensantenna in which the propagation characteristic may be changed by changing the mesh size or, what amounts to the same thing, the effective size of the conductive wire of the wire-grid, either alone or in combination with changing the distance of separation between the spaced wire-grids.

Other objects and a better understanding of the invention may be had by reference to the following description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a perspective view of a wire-grid lens antenna constructed in accordance with this invention in which a desired pointto-poin't' change of the propagation characteristic is obtained by changing both the point-to-point spacing between and the mesh size of opposite wire-grids;

FIG. 2 is an enlarged top view of the lens portion of the antenna shown in FIG. 1 in which the upper wire-grid exactly overlies the lower wire-grid;

FIG. 3 is an enlarged perspective view of a pair of overlying meshes constructed of compound wires;

FIGS. 4, 5 and 6 are sectional views taken respectively along lines 44, 55, and 66 of FIG. 2;

FIG. 7 is a diagram showing the equivalent static dielectric constant e of a grid-wire lens constructed with square meshes of two-strand conductors versus the ratio of grid to ground spacing a to mesh size 12 for different values of the ratio of conductor-to-conductor spacing d to mesh size b, for

where r is the radius of the single-strand conductor.

FIGS. 811 and 8b are illustrative of the change from a two-strand square-mesh to a one-strand square-mesh of half the original size;

FIGS. 9a and 9b are illustrative of the change from a three-strand mesh-grid to a one-strand mesh-grid of onethird of the original size; and

FIGS. 10, l1 and 12 are alternate embodiments of multistrand arrangement of compound wires for the construction of meshes for practicing this invention.

Referring now to the drawings, and particularly FIG- URE 1 thereof, there is shown a wire-grid lens antenna 20 constructed in accordance with this invention and comprising a center portion 21 forming a wire-grid lens for azimuthal beam shaping and a peripheral portion 22 forming a radiating structure for elevational beam shaping and for matching the impedance between wire-grid lens 21 and the surrounding space. Radiating structure 22 is shaped in the form of a biconical or radially flared horn.

Wire-grid lens 21 includes an upper wire-grid 23 and a lower wire-grid 24 spaced in opposite and overlying relation by means of a plurality of non-conductive suspension or support members 25. As a practical matter the peripheral portion of upper wire-grid 23 and lower wiregrid 24 may be secured to rim members such' as a pair of aluminum rings 26, 27 respectively, which are light in weight and which form convenient conductive terminating and support edges. Furthermore, aluminum rings 26, 27 also provide a convenient means for attaching radiating structure 22 thereto. Support members 24 may comprise Wooden, plastic or fiber glass poles anchored by means of guy wires having appropriate insulating members 28 inserted therein.

As will be explained in greater detail hereinafter, lens wire-grids 23 and 24 are formed, either throughout or in part, of composite metallic wires, that is, wires having two or more single-strand conductors substantially parallel to one another and lying in the surface defined by the respective wire-grids. The mesh structure formed by the compound Wires may have a variety of different geometric mesh' shapes such as square, triangulaljor regular hexagonal. Even ,though different geometrically shaped meshes may be employed, it is desirable to utilize the identical mesh structure throughout a particular lens, particularly in those portions of the lens in which the spacing between opposite wire-grids is less than the mesh size as explained fully in my previous referred-to copending application. Furthermore, the meshes in the closely spaced portions, Whether single or multi-strand wires, are preferably oriented with respect to one another in such a manner that corresponding sides of opposite meshes, that is opposite compound wires (and their singlestrand conductors) form respectively two-wire transmission lines. I

Upper and lower grids 23, 24 may be regarded as a pair of conductive surfaces having a predetermined pointto-point spacing. Even though both wire-grids are shown to be convex as seen from inside the lens having identical curvature, they may take on diiferent geometrical con-- For example, the lens of this invention may figurations. be formed of a planar surface and a convex surface or of two convex surfaces of dilferent curvature or of a concave surface or any combination thereof. As long as the pointto-point spacing is selected to provide the proper point-topoint wave propagation velocity, the surfaces may take on any form. Also, incertain embodiments one of the wire-grids may be entirely omitted and replaced with a solid conductive surface such as the conventionally known ground plane which provides mirror symmetry.

Referring now to FIGURE 2, there is shown lens 21 mad up of overlying lens wire-grids 23 and 24 so that only upper wire-grid is visible. Wire-grid 23 is formed of a plurality of substantially orthogonal composite wires 30 and 31 respectively, arranged as an interlacing weave and warp. Each composite wire 30, 31 consists of a pair of single-strand conductors respectively designated by reference characters 32, 33 for composite wire 30 and 34, 35 for composite wire 31.

The single-strand conductors of each composite wire are connected conductively at each cross-over point with each crossing single-strand wire to form connection such as designated by reference character 36 in FIGURE 3. In this embodiment both single-strand conductors of a composite wire always lie in the surface of the wire grid.

It is not necessary, however, that all single-strand conductors utilized to make up the composite wire lie in a common plane. They may, in fact, lie on opposite sides of an effective grid surface as shown in FIGURES 11 and 12.

In'the center portion 40 of wire-grid 23, wires 30, 31 are either single-strand conductors or closely-spaced multiple-strand conductors which are substantially paral-' lel to one another. Towards the peripheral portion 41 wires 30, 31 become double-strand conductors 32,. 33, 34 and 35 which increase their distance of separation as they approach ring 26. As will be explained hereinatfer, the greater their separation, the greater is the velocity of wave propagation for a constant spacing of opposite wire-grid.

In order to indicate where central lens portion 40 ends and peripheral lens portion 41 begins, a circular dash-dot line 43 is provided. In practicing the invention, indicating line 43 may have any desired radius with regard to lens 21. In fact, if the radius of indicating line 43 is equal to that of lens 21 then the embodiment of the lens in my. co-pending application is obtained in which the conductor separation. is either zero (single wire) or constant throughout. In other words, indicating line 43 designates the portion of lens 21 at which the wave velocity of propagation is controlled, at least in part, by changing the effective diameter of compound wires 30, 31 or what amounts to the same thing, the mesh size.

Referring now to FIGURE 3, there is shown a single mesh of a double-strand wire such as wire 30, 31. The upper mesh, which forms a portion of grid 23, comprises; substantially parallel orthogonal single-strand conductors 32, 33, 34 and 35, connected in parallel at junction points 36. The lower mesh, which forms a portion of lower wire-grid 24, is substantially identical thereto and is made up of substantially orthogonal single-strand conductors 44, 45, 46, and 47.

If the radius of each single-strand conductor, such as conductor 32, is equal to r and the separation between adjacent conductors is dlff the combination thereof (forming compound wire 30) may be considered to have an equivalent radius which is equal to the geometric mean of the radius of each conductorand their spacing, provided the spacing d is large compared with "r' but smali compared with b. Supposing that conductor radius r{ is equal to 1 millimeter and their separation d is equal: to 10 millimeters, the equivalent radius of the compound wire would be equal to 3.16 millimeters. If the separa-' tion d is increased to millimeters the effective radius of the compound wire would be equal to 10 millimeters.

The basis for the foregoing equivalent radius dcterminations, may be found in a book entitled Inductance Calculations, Working Formulas and Tables by Frederick W. Groover, PhD., published by the Dover Publication, copyright 1946, on page 35, commencing with Formula 7 and extending through page 37, Formula 12.

As shown in a paper entitled Properties of a Pair of Wire-Grids for Use in Lens HF Antenna by Andreasen and Tanner, 1961, Western Electronic Show and Convention, Paper #1/ 3, the index of refraction or the permittivity of a' wave travelling between wire-grids is, among other variables, a function of the radius of the wire defining the mesh and decreases with increasing radius. Accordingly, the wave may be speeded up by increasing the effective radius of the compound wire, i.e., by increasing the distance of separation d between the single-strand conductors forming the compound wire.

In addition to the theoretic explanation given in the: above referred to paper, the increase of wave propagation velocity with increase of separation of adjacent con-- ductors is easily understood on the basis of premises setv forth in my copending application. As shown therein, the wave propagation velocity depends on the spacing between opposite wire-grids. For a spacing of about four times the mesh size the wave propagation velocity is equal to that of light (free space). For smaller spacing, the wave propagation velocity decreases. Separation of the single-strand conductors is very much akin to a decrease in the mesh size. Since the wave propagation velocity depends on the spacing between opposite wiregrids in terms of mesh size, any'decrease of mesh size is equivalent to an increase of spacing and consequently results in an increase in the wave propagation velocity.

A qualitative explanation of the foregoing is as follows. Assume that the spacing between the grids is a small fraction of the wavelength, therefore, a wave propagating in the region between the grids will radiate very little. If this spacing is much larger than the size of the grid mesh, the field of a quasi-plane wave propagating between the grids is nearly homogeneous and the wave velocity is nearly the velocity of light. When, on the other hand, the spacing between the grids is much smaller than the mesh size, the field of a quasi-plane wave is confined to a region very close to the wires. This has the effect of reducing the wave velocity to 1 /2 times the velocity of light.

This can be understood by visualizing a spaced pair of square mesh wire grids. When the spacing between these grids is much smaller than the mesh size and a wave is propagating along one set of wires, the crossed wires merely double the capacitance of the wires along which the wave is propagating. At the same time, the crossed wires have only a minor effect upon the inductance of the wires. Thus, the wave velocity is reduced by a factor of nearly the square root of two. Since the mesh size is assumed to be electrically small, the grid pair is substantially isotropic. The same Wave velocity will therefore be found for a wave moving along the diagonal directions of the mesh grids. This occurs because as the wave energy follows the mesh wires along the diagonal direction, there is very little energy storage and thus the effect of velocity of the wave is smaller than the velocity of 'light by a factor equal to the ratio of twice the mesh size to the mesh diagonal or the square root of two. In other words, for this situation, the mesh structure represents simply a meander line for a wave propagating in the diagonal direction of the grids and the wave is constrained to travel an actual distance greater by the square root of two than the effective diagonal propagation distance so that the wave is slowed down by this factor. In the situation where the spacing between the grids is large relative to the mesh size, even though the mesh size of the grids is small compared with the wavelength, the fields in the region between the grids are approximately homogeneous and the grids act essentiallyas metal plates, and the wave in the region between them propagates at nearly the velocity of light.

Referring now to FIGURES 4, 5 and 6 which show sections taken respectively along lines 44, 5-5 and 6-6 of lens 21, it can be seen that in center portion 40 (between lines 5050) both upper and lower grids 23 and 24 are convexly curved toward one another and wires 31 are formed of single-strand conductors (or possibly two-strand conductors very closely spaced with a constant small distance of separation d). Lines 50-50 corresponds to indication line 43 and delineates center portion 40' in which wave velocity control is provided by Chang ing the grid to grid spacing a.

In peripheral portion 41, the wave velocity of propagation, as already stated, is controlled by changing the effective equivalent radius r of conductors 31 either solely or in combination with changing the grid spacing a. As a practical matter it has been found that a com bination of these two methods of controlling the wave velocity of propagation accomplishes the object of keeping the maximum peripheral separation a within proper limits without unduly complicating the mesh structure of grid 23 and 24.

To provide wave velocity control entirely by increasing the effective equivalent radius of wires 31 for a lens whose center portion 40 is of the same order of magnitude as peripheral portion 41 (similar radial'dimensions) multistrand wires having more than three conductors would be necessary.

In viewing FIGURE 4 it is immediately apparent that the spacing d, between adjacent conductors 34, 35, increases with increasing radial distance from the center of lens 21. Similarly, spacing a between grids 23, 24 increases with increasing radial distance, but much less rapidly than was necessary in practicing the invention disclosed in my co-pendingapplication. For comparison purposes, dotted lines 51 and 52 are shown which represent the required grid separation inthe absence of increasing the effective equivalent wire radius, that is'lines 51 and 52 represent a lens constructed in accordance with the wire-grid lens antenna disclosed "in my above-referred to copending application.

FIGURES 5 and 6 are further sectional views showing the change 'ofconductor spacing 'd at two off-set positions with increasing radial distance. In FIGURE 5 center portion 40, indicated by lines 5050, is of course smaller and conductor 34, 35 separation is the same as in FIGURE 4. In FIGURE 6 the off-set of line 66 is greater than theradius of center portion 40 so that separation between conductors 34, 35 is evident all along this view.

FIGURE 7 shows a graph in which the effective dielectric constant e is plotted against the ratio of the spacing a of a wire-grid from a ground plane to mesh size b for a square mesh two-conductor compound wire for a grid having a ratio of mesh-size b to radius of a singlestrand conductor r equal to 1000. The effective dielectric constant e is given for several different values of the ratio of conductor spacing d to mesh size 12. Curves 70, 71, 72 and 73 respectively show ratios of 0.01, 0.1, 0.2 and 0.5. It is immediately apparent from the graph that the effective dielectric constant 6 decreases (and thereby the wave velocity increases) for increasing conductor spacing d. It is further seen that for a constant conductor spacing, curves 70-73 follow curve 74 which represents a single-strand wire mesh such as shown in my co-pending application.

FIGURESSa and 8b respectively show the conversion of four two-strand compound wides 80, 81, 82 and 83 made up of conductors 80a, 80b, 81a, 81b, 82a, 82b, 83a and 83b and arranged to form a square mesh into a smaller mesh structure by increasing the spacing conductor spacing to one-half of the original mesh size. As

is immediately seen, the effect of such conductor spacing made up of conductors a, 90b, 90c, 91a, 91b, 91c, 92a,

92b, 920, 93a, 93b and 93c and arranged to form a square mesh into a smaller mesh structure by increasing the conductor spacing to one-third of the original meshes size. As is immediately seen the effect of such conductor spacing is the same as decreasing the mesh size by a factor of three.

Obviously, the above explanation is equally applicable to four-strand and higher number of strand compound wires which upon separation decrease the mesh size by a factor of four or more depending on the number of conductors forming the compound wire.

FIGURES 10, 11 and 12 show, by way of example, two further embodiments of separating multi-strand conductors to increase the wave velocity of propagation. In each figure, the basic mesh size, that is the mesh size prior to spreading the conductors is indicated by Parallel lines 102 and 103. In other words, the basic mesh size in terms of compound wires is the distance between lines 102 and 103. Similarly, the upper and lower effective grid surface is indicated by lines and 101 respectively.

FIGURE 10 shows an upper pair of two-strand wires comprising conductors 110a and 1101) and a lower pair of two-strand wires compriisng conductors 111a and 111b being spread in the plane of the respective upper and lower effective grid surfaces 100 and 101 in the manner shown in FIGURES 2, 3 and 4.

FIGURE 11 shows an upper pair of two-strand wires comprising conductors 112a and 1121) and a lower pair of two-strand wires comprising conductors 113a and 1131b being spread in aplane vertical to respective upper and lower effective grid surfaces 100 and 101. This embodiment of increasing the effective equivalent wire radius may be utilized to provide substantially the same Wave velocity reduction as the embodiment shown in FIGURE 10 providing the conductor spacing is the same.

In fact, spreading of multi-strand wires may be provided not only in the orthogonal planes shown respectively in FIGURES 10 and 11, but along other planes. Also in case of more than two-strand wires, the several conductors may be arranged in a multitude of different patterns, one of which is shown, by way of illustration in FIGURE 12.

In FIGURE 12, an upper pair of three-strand wires comprising conductors 114a, 114b and 114c and a lower pair of three-strand wires comprising conductors 115a, 115b and 115a are spread to form the corners of an equilateral triangle. Similarly four-strand wires may be spread to form the corners of a square or in a single plane or in many other ways. Each such configuration provides an increase of the wave velocity of propagation with increased conductor spacing.

There has been described an improved grid-wire lens antenna in which the point-to-point wave velocity of propagation is controllable not only by grid-to-grid spacing, but also by conductor-to-conductor spacing of the multi-strand wires forming the basic mesh structure of the grids. As a result of such control, the grid separation may be kept at a minimum.

What is claimed is:

1. In an antenna structure, a wire-grid lens comprising a pair of overlying surfaces, at least one of said surfaces being formed of a wire-grid having a mesh opening size which is small in comparison with the shortest operating wavelength of said antenna structure and having certain meshes toward the outer perimeter of said surface formed of spacing apart multi-strand conductors.

2. In an antenna structure, a wire-grid lens comprising a lens portion formed of a pair of overlying conductive wire-grids having a mesh opening size which is small in comparison with the shortest operating wavelength of said antenna structure, at least a certain portion of the meshes of said overlyingWire-grids being formed of at least two single-strand conductors which are spaced from one another a distance which is small compared to the mesh opening size but large compared to the radii of said conductors.

3. In an antenna structure, a wire-grid lens comprising a pair of overlying, spaced, wire-grids having a mesh size substantially smaller than the shortest operating wavelength of said antenna structure of said wire-grids including at least a portion in which the sides of said meshes are formed of compound wires including at least two spaced single-strand conductors extending substantially side-by-side, the spacing between said conductors increasing from a predetermined minimum value with increasing wave propagation velocity, but not exceeding a spacing which is small compared to said mesh opening size.

4. In an antenna structure, a wire-grid lens comprising a pair of overlying, conductive, spaced wire-grids, each wire having meshes formed of composite wires and each mesh being substantially smaller than the shortest operating Wavelength of said antenna structure, each composite wire including at least a pair of single-strand conductors lying in the surface defined by its associated wire-grid and extending substantially side-by-side in spaced relation, said pair of conductors being spaced apart a predetermined distance for establishing a predetermined wave velocity of propagation.

5. In an antenna structure, a wire-grid lens comprising apair of overlying, conductive, surfaces having a predetermined, smoothly varying spacing therebetween, at least one of said surfaces being formed of a wire-grid having a mesh size which is substantially smaller than the shortest operating Wavelength of said antenna structure, at least a portion of said wire-grid having meshes whose sides are formed of composite wires, said composite wires including a plurality of spaced single-strand conductors electrically interconnected at each crossover point with like conductors extending in the same plane, said conductors being spaced at each point of said wire-grid a predetermined distance to provide a selected wave velocity of propagation at each of said points.

6. A wire-grid lens in accordance with claim 5 where in the spacing between said overlying surface is selected from a distance of approximately one-tenth of the mesh size to provide a wave propagation velocity substantially equal to x/l/Z of the free space wave propagation velocity to a distance which is less than four times the mesh size and in which the spacing between said conductors is increased in the portion of increased Wire-grid spacing to provide a wave propagation velocity substantially equal to the free space wave propagation velocity.

References Cited by the Examiner UNITED STATES PATENTS 2,485,138 10/1949 Carter 343-815 2,511,916 6/1950 Hollingsworth 333- 2,75 6,424 7/ 1956 Lewis 343-909 3,047,860 7/1962 Swallow 343- 897 FOREIGN PATENTS 100,539 7/ 1925 Austria.

OTHER REFERENCES Silver: MIT Rad. Lab. Series, vol. 12, page 449 relied upon.

ELI LIEBERMAN, Acting Primary Examiner. 

1. IN AN ANTENNA STRUCTURE, WIRE-GRID LENS COMPRISING A PAIR OF OVERLYING SURFACES, AT LEAST ONE OF SAID SURFACES BEING FORMED OF A WIRE-GRID HAVING A MESH OPENING SIZE WHICH IS SMALL IN COMPARISON WITH THE SHORTEST OPERATING WAVELENGTH OF SAID ANTENNA STRUCTURE AND HAVING CERTAIN MESHES TOWARD THE OUTER PERIMETER OF SAID SURFACE FORMED OF SPACING APART MULTI-STRAND CONDUCTORS. 